LM2700_04 NSC [National Semiconductor], LM2700_04 Datasheet - Page 3

• Manufacturer Part Number: LM2700_04
• Description: 600kHz/1.25MHz, 2.5A, Step-up PWM DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

Block Diagram
Detailed Description
The LM2700 utilizes a PWM control scheme to regulate the
output voltage over all load conditions. The operation can
best be understood referring to the block diagram and Figure
1 of the Operation section. At the start of each cycle, the
oscillator sets the driver logic and turns on the NMOS power
device conducting current through the inductor, cycle 1 of
Figure 1 (a). During this cycle, the voltage at the V
controls the peak inductor current. The V
crease with larger loads and decrease with smaller. This
voltage is compared with the summation of the SW voltage
and the ramp compensation. The ramp compensation is
used in PWM architectures to eliminate the sub-harmonic
oscillations that occur during duty cycles greater than 50%.
Once the summation of the ramp compensation and switch
voltage equals the V
C
voltage, the PWM comparator resets
C
voltage will in-
C
pin
3
the driver logic turning off the NMOS power device. The
inductor current then flows through the schottky diode to the
load and output capacitor, cycle 2 of Figure 1 (b). The NMOS
power device is then set by the oscillator at the end of the
period and current flows through the inductor once again.
The LM2700 has dedicated protection circuitry running dur-
ing normal operation to protect the IC. The Thermal Shut-
down circuitry turns off the NMOS power device when the
die temperature reaches excessive levels. The UVP com-
parator protects the NMOS power device during supply
power startup and shutdown to prevent operation at voltages
less than the minimum input voltage. The OVP comparator is
used to prevent the output voltage from rising at no loads
allowing full PWM operation over all load conditions. The
LM2700 also features a shutdown mode decreasing the
supply current to 5µA.
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